linfa-preprocessing::countgrams: RefCell is replaced with OnceLock

[kakserpom]

Simple fix to allow for concurrent access.

[relf]

OnceLock is not serde. What about cloning to use multi-threading?

[kakserpom]

@relf I don't quite understand what you mean by that. OnceLock is a better fit here than RefCell because it's thread-safe.

[relf]

With OnceLock we cannot serialize/deserialize CountVectorizerValidParams which is a no go (cf. CI tests errors).

I meant you can just clone CountVectorizerParams and use an independent copy in each thread (if it was your initial problem with the multi-threading).

[kakserpom]

@relf Well, I guess we could solve the issue by implementing serialize and deserialize manually, right?

[relf]

Maybe... But there is no problem in the first place if you just clone CountVectorizerParams so no need of a special serde code complication.